Contributed by Ross Burhouse

The highly anticipated Metro Silver Line Phase 2 will span 11 miles, including six new stations, and will connect Reston, Virginia, to Dulles Airport and beyond to the growing suburban areas of Loudoun County, Virginia. Before a single piece of track could be laid, our design-build team, Capital Rail Constructors, partnering with contractor joint venture Clark Construction and Kiewit Infrastructure and lead designer Parsons Transportation Group, began developing steel rail bridges to support the new transportation infrastructure. The design-build team had to contend with tight spaces, long bridge spans to accommodate future road development underneath, as well as work within Washington Metropolitan Area Transit Authority's (WMATA) design criteria.

The result is three bridges with unique designs that incorporate innovative engineering solutions. I co-authored a paper on the Dulles Metrorail steel bridges for the 2018 World Steel Bridge Symposium. After decades of planning, the Silver Line Phase 2 is currently slated to open in 2020, and our team's steel bridges will help support this critical piece of infrastructure.

Thru-Girder Bridge Solutions

One bridge, at Centreville Road, needed to accommodate future expansion of the roadway underneath. The preliminary engineering drawings provided by the Metropolitan Washington Airports Authority (MWAA) presented a possible solution that used temporary piers that would have had to been removed when Centreville Road was widened.

As an alternative, the design-build team proposed to cross Centreville Road with a single span of 150 feet, which would not require any future modifications to the superstructure for the roadway widening. Our design used a thru-girder bridge, with the girders on either side of the tracks rather than below the deck, allowing for more clearance over the roadway. This innovation allowed for a substantial reduction in the height of approaches required for the bridge.

Obtaining approval from MWAA and WMATA for a design that differed significantly from the preliminary drawings was a challenging process. But, once the alternate design was approved for the Centreville Road crossing, we were able to use the same thru-girder concept for the bridge at Horsepen Run, which crosses the stream with a single span of 130 feet and eliminates the need for any piers.

Bridges for Tight Spaces

The bridge over Broad Run presented another challenge. This bridge was constructed in the tight space between two existing highway bridges carrying the Dulles Greenway. While construction of the rail infrastructure had been planned, right-of-way was reserved at the bare minimum for a rail bridge to run in between—as little as two feet of clearance remained on either side.

To satisfy hydraulic requirements the new piers had to be aligned with the adjacent existing piers and match the center span length of nearly 180 feet. WMATA requires special approval for spans over 150 feet, so design of this structure drew extra scrutiny so that the longer span satisfied design criteria.

The limited space available between the Greenway bridges also presented challenges in erecting the girders, since the adjacent bridges could not support the loading from a crane to erect the new structure.

Ultimately, the three span bridge had to be built in stages. First, the piers were constructed, and the center span girders were erected using cranes positioned between the Greenway bridges on either side. Then, the cranes were removed, the abutments constructed, and the end span girders erected by cranes located behind the abutments.

Design-Build Advantages

This project is a significant one for the Washington, D.C. metro area because of the accessibility it will bring to thousands of residents in Northern Virginia. It was also a significant example of the efficiency and creativity of design-build teams to reach innovative solutions. The design-build process and collaboration between contractor and engineer, has enabled an extended Metrorail system that will benefit commuters and Dulles Airport travelers for years to come.